>...you rotate the spare tire with the regular tires? ... why?
Several reasons, assuming you have a full size spare on a matching wheel;
Logically, if 4 tires last for 80,000 miles, 5 will go 100K, as each tire "rests" in the trunk 20% of the time.
If you don't rotate the spare, you'll end up trading/selling the car with a new spare for no monetary return.
If the spare never leaves the trunk, you could find it flat when you actually need it.
It gives you a look in the trunk wheel well for possible water accumulation.
It's virtually the same amount of work to change 5 as 4, you still re & re 4 sets of bolts.
(The first 2 are possibly my Dutch heritage.
BTW, the word is "Frugal", not "cheap" :0)